1. Field of the invention
The invention relates to an exclusive control unit and particularly to an exclusive control unit for performing an exclusive control to be performed among a plurality of computers For a shared resource, for example, a database stored in a disk unit and the like shared among a plurality of computers.
2. Description of the Related Art
In recent years, with downsizing of computers, technologies for operating a database with increased security, at a higher speed, and at a higher net working rate on computers called workstations, have been developed. As an approach for solving these problems in database services, Oracle Inc. has provided a product called Parallel Server. This is a database program to be run on a plurality of computers linked to one another on a network, and the database program is run in parallel on the respective computers in relation to the same database shared among the plurality of computers.
This program has improved a database service in response speed and furthermore in net working rate, by means of parallel running, since the database service is maintained even if a failure happens in any of the computers running on the same database by the database program being run on other computers that are not suffering from problems.
In the case of running a plurality of database programs for the same database shared among a plurality of computers, an exclusive control is needed in order to avoid a simultaneous update operation to the same data. Oracle Inc. or another system vendor such as Pyramid Inc. has implemented and provided such an exclusive control mechanism in the shape of software for performing data exchange through a network. However, such an implementing method by means of software is liable to be affected in its performance by such factors as network traffic, average load on computers, and the like, and its cost for the exclusive control is a considerable burden to the overall performance of the database system.
As mentioned above, an exclusive control is needed among a plurality of computers running on the same database, and in the case of implementing an exclusive control mechanism by means of software, its performance becomes a problem.
By the way, an exclusive control when a resource is shared among a plurality of computers can be performed not only by software, but also hardware. For example, Japanese Patent Application Laid-Open 364514-1992 discloses a case in which each of a plurality of computers composing a multi-host system accesses a device on a Small Computer Systems Interface (SCSI) bus composed for the multi-host system. The SCSI protocol primarily has a specification to make it possible to connect a plurality of computers with one another and its exclusive control is performed by inputting SCSI commands called Reserve and Release, which are inserted between other commands for accessing data.
The Japanese Patent Application Laid-Open 364514-1992 discloses that an exclusive controller provided in parallel with a SCSI bus solves a cost problem of the Reserve and Release and a problem that states of computers for exclusive control of the computers in a multi-host system become contradictory to one another when a reset operation is performed by either of target controllers after some host computer has issued a Reserve command, since the SCSI specification gives freedom of resetting the bus to each of its target controllers.
Further, Japanese Patent Application Laid-Open 148365-1988 discloses a method for performing an exclusive control for data stored in a disk shared between two computers by operating on the data in a shared memory disposed between the two computers. However, in an exclusive control mechanism of the prior art by means of software and data exchange using a network which is generally adopted in the case that an exclusive control is needed among a plurality of computers, its performance often becomes a problem.
Also, in an exclusive control mechanism of the prior art by means of data on a shared memory of a shared memory unit disposed between computers, there is a problem that an exclusive control process cannot be maintained in a case where a problem occurs in the shared memory.
Further, since the shared memory unit is provided to a system as a dedicated hardware component, there is a problem that the cost of the system becomes expensive. This problem is the reason why an exclusive control system by means of software and data exchange through a network is often used in spite of its performance problem.
In a method using one shared memory, in which an exclusive control is needed, there is also a problem that an increase in the number of trials for acquiring an occupancy right for occupying a resource, which is caused by an increase in the number of computers sharing the resource, becomes a great burden to the system performance.
Another problem to be solved by the invention is to secure a communication path among computers. For example, in the case of composing a multiplexing system by means of personal computers, a multiplex communication path of high reliability is needed to prevent a single-point failure on the communication path.